Aid deliveries by ship were impossible to Port-au-Prince because the Haitian capital’s port was closed due to severe damage from Tuesday’s magnitude-7 earthquake. The city’s airport was open but damaged, laboring mightily to handle a flurry of incoming aid flights.

An estimated 45,000-50,000 were killed, based on government figures and a wide network of Haitian volunteers across the hard-hit capital Port-au-Prince, said Red Cross federation spokesman Jean-Luc Martinage.

Fearful of going near quake-damaged buildings, Haitians stood or rested on the roads, slowing the transport of food and other crucial aid.

Coordinating deliveries was also a problem, which is being tackled by U.N. and U.S. officials, Byrs said.

With the U.N. peacekeeping force in tatters, representatives of aid organizations say there does not appear to be anyone coordinating distribution of relief supplies at the airport in Port-au-Prince.

“It is difficult because folks at the Port-au-Prince airport are trying to get up to speed to run logistics,” said Save the Children spokeswoman Kate Conradt from Haiti.

“Donations are coming in to the airport here, but some are coming without notice from very well-meaning groups,” she said. “There is not yet a system to get it in.”

Severe damage to at least eight Port-au-Prince hospitals made it nearly impossible to treat the thousands of injured or prevent outbreaks of disease, said Paul Garwood, spokesman for the World Health Organization.

Haiti, the poorest country in the Western Hemisphere, presents unique logistical challenges for aid workers even in the best of times. It shares an island with the Dominican Republic, meaning that aid must arrive by sea or air. Haitian streets are in poor condition under normal circumstances, and even if aid reaches the Dominican Republic, the road from there to Port-au-Prince is narrow and easily clogged.

Almost everything has to be imported, even wood for building temporary shelters, because Haitians have denuded their hillsides by cutting trees for cooking fuel.

“If you see Dominican Republic and Haiti from the air, it’s really striking,” said Byrs. “Half of the island is green and the rest is dust.”

In addition, Haiti was already heavily damaged by a series of severe hurricanes, the most recent in 2008.

President Barack Obama announced Thursday the U.S. government was making an initial $100 million relief effort and promised an all-out rescue and humanitarian effort that included military and civilian emergency teams from across the U.S.

“We have to be there for them in their hour of need,” Obama said.

The aircraft carrier USS Carl Vinson was deployed to Haiti, and the amphibious assault ship USS Bataan had been ordered to sail as soon as possible with a 2,000-member Marine unit.

Even as the United Nations stepped up its massive aid operation, the world body was trying to determine how many of its own staff were killed in earthquake.

“It’s very difficult to give an exact number,” Byrs said. “This is also a tragedy for the United Nations.”

She said up to 100 U.N. staff were trapped in the main U.N. peacekeepers’ building, which was destroyed.

Byrs said the U.N. believes there is a good chance many people are surviving in the rubble and might be saved.

She said 40 search-and-rescue teams from around the world had started arriving in Haiti to look for survivors trapped inside collapsed buildings. But to find and save people, the rescuers need heavy machinery to lift tons of rubble – equipment that teams from places like Britain and Iceland have, but others don’t

Haiti has virtually none of those machines but aid workers were trying to get some into Haiti from the Dominican Republic, Charles Vincent of the World Food Program said.

“We’ll have to see how that works out,” said Vincent. “The U.S. military will also be bringing in some equipment.”

The desperate situation has aid groups fearing a surge in lawlessness, Vincent said. U.N. peacekeepers are patrolling to try to control looting but they are dealing with many deaths and injuries of their own, he said.

The International Committee of the Red Cross said its forensic specialists would help ensure that bodies of the quake victims are recovered and identified for the benefit of their families.

The Red Cross set up a special Web site to help Haitians find their missing loved ones, and after just a few hours, over 5,000 people had already registered on it, many from the United States and Canada.

Aid was delivered or promised from many countries, including Brazil, the European Union, Britain, Germany, Israel, France, Switzerland, South Korea and Canada. China dispatched a chartered plane carrying 10 tons of tents, food, medical equipment and sniffer dogs, along with a 60-member earthquake relief team who worked in China’s own 2008 earthquake, which killed some 90,000 people.

The Red Cross estimated that some 3 million people in Haiti will require aid, ranging from shelter to food and clean water, and said many Haitians could need relief aid for a full year.

Aid workers base such estimates on a formula that has been refined over the years as disasters have been studied, said ICRC spokesman Florian Westphal.

“There is pretty much a standard formula, but even that will vary. With drinking water, for example, you know that people need at least 15 liters a day, but there are situations where that’s not possible so there is a period in which people can survive with 6 liters a day for a limited amount of time.”

The ICRC sent out a kit of medical material Thursday, which he said was supposed to be enough for 10,000 people for three months to cover basic diseases and medical conditions. “There is clearly an overall calculation template behind that,” he said, but added that “it may turn out that there is a huge outbreak of disease and the supplies don’t last anywhere near three months.”

Agencies use the so-called “Sphere” handbook that gives guidelines for how best to respond to humanitarian emergencies.

“A lot depends on local assessment,” Westphal said. “For shelter, what they’re going to do now is look at a limited area and see how many houses are destroyed, and extrapolate that for the whole affected area. It’s a combination of established standards and local assessment.”

He said aid specialists make a continuous effort to learn from every single crisis.

“These operations are so complex,” Westphal said. “We have to be realistic. There will be mistakes, and the only way we can get better is by analyzing and learning from previous disasters.”

Pablo Medina, operations coordinator of the International Federation of Red Cross and Red Crescent Societies, said, “At this very early hour, with such limited amount of information, what you have to do is base your calls on past experience on previous earthquakes, on media reports and on information on the ground,” Medina told the AP.

Initial planning is conservative and is normally revised upward as more information becomes available. This time, the Red Cross decided to send 100 experts to Haiti.